A multiple-input multiple-output (MIMO) scheme of a wireless communication system is a transmitting/receiving method using a plurality of transmitting antennas and a plurality of receiving antennas. In the MIMO system, a plurality of radio channel paths are generated between transmitting and receiving antennas, and transmitting/receiving ends separate them or combine them to increase data transmission capacity or improve transmission quality. The MIMO scheme includes a spatial multiplexing scheme and a spatial diversity scheme. A downlink MIMO scheme introduced to a long term evolution (LTE) system includes transmit diversity, cyclic delay diversity (CDD), beamforming, and spatial multiplexing schemes. Also, a multiuser MIMO (MU-MIMO) scheme for simultaneously transmitting data to a plurality of terminals from an identical resource is supported.
An antenna port of the LTE standard is a logical antenna unit realized by a weighted sum of one or a plurality of physical antenna elements, and is generally defined by the transmitting end. The antenna port is a basic unit by which a reference signal (RS) is transmitted. Therefore, a terminal estimates a channel not for the physical antenna element but for each antenna port, and measures and reports channel state information (CSI) based upon it. Different antenna port numbers are assigned to a cell-specific RS (CRS), a user equipment-specific RS (URS), and a CSI-RS that are LTE downlink reference signals, respectively. A purpose of the URS is to decode a physical downlink shared channel (PDSCH) of the terminal, so the URS is also called a demodulation RS (DMRS). The antenna port number for the CRS may be 0 to 3, the antenna port number for the URS may be 7 to 14, and the antenna port number for the CSI-RS may be 15 to 22. Mapping between the antenna port and the physical antenna element(s) is referred to as antenna virtualization. The terminal may not basically know which virtualization is applied to each antenna port.
The CSI-RS is a downlink reference signal transmitted by a base station so that the terminal may acquire CSI, and it is introduced in LTE Release 10. The CSI-RS is also referred to as a non-zero-power (NZP) CSI-RS in order to distinguish it from a zero-power (ZP) CSI-RS to be described. In the existing Release 8/9 system, the CRS is used to acquire CSI of the terminal, and starting from Release 10, an introduction of a new reference signal for channel estimation with lower density than the existing CRS is needed so as to support downlink transmission of up to 8 layers. CSI-RS configuring information is transmitted to the terminal through user equipment-specific radio resource control (RRC) signaling. Numbers of CSI-RS antenna ports configurable for the terminal are 1, 2, 4, 8, 12, and 16 up to the present Release 13. Regarding the number of CSI-RS antenna ports, numbers of total REs occupied by transmission of CSI-RS per pair of physical resource blocks (PRBs) are 2, 2, 4, 8, 12, and 16.
A transmission period of CSI-RS on a time axis may be configured as 5, 10, 20, 40, or 80 ms. According to the present standard, each CSI-RS antenna port has a gap of 12 resource elements (REs) on a frequency axis.
In order for a base station to perform three-dimensional beamforming in a full dimension (FD)-MIMO (or three-dimensional MIMO) system, the terminal needs to measure and report CSI on a vertical axis in addition to existing CSI on a horizontal axis. Methods for increasing the number of CSI-RS antenna ports configurable to the terminal when a size of two-dimensional antenna array is large have been researched.
Further, the existing terminal only recognizes the CSI-RS antenna port array as one dimension. Methods for the terminal to recognize the CSI-RS antenna port array as two dimensions (2D) or three dimensions (3D) are necessary.